An ink-jet printer includes a pen in which small droplets of ink are formed and ejected toward a printing medium. The pen is mounted to a reciprocating carriage in the printer. Such pens include printheads with orifice plates having very small nozzles through which ink droplets are ejected. Adjacent to the nozzles are ink chambers where ink is stored prior to ejection. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply. The ink supply may be, for example, contained in a reservoir section of the pen or supplied to the pen from a remote site.
Ejection of an ink droplet through a nozzle may be accomplished by quickly heating a volume of ink within the adjacent ink chamber. The thermal process causes ink within the chamber to superheat and form a vapor bubble. Formation of a thermal ink-jet vapor bubble is known as "nucleation." The rapid expansion of ink vapor forces a drop of ink through the orifice. This process is called "firing." Ink in the chamber may be heated, for example, with a resistor that is responsive to a control signal. The resistor is aligned adjacent the nozzle.
Ink-jet printers are affected by fluid pressure changes within the printer system. An undesirably high fluid pressure may cause ink to flow uncontrollably to the printhead, subsequently forcing ink through the nozzles. Ink leakage through the printhead nozzles is known as drooling.
Irrespective of whether there is a substantial increase in fluid pressure within the printer, it is desirable to establish a slight back pressure within the system. The presence of a back pressure ensures ink is expelled only when the printhead is activated (i.e., when ink is fired). As used herein, the term "back pressure" means a partial vacuum within the printhead. Back pressure is considered in the positive sense, so that an increase in back pressure represents an increase in the partial vacuum. Accordingly, the back pressure is measured in positive terms, such as water column height.
Although previous ink-jet pens have incorporated a pressure regulator on the pen, these regulators were large and heavy causing a decrease in print speed. Thus, conventional ink-jet pens are sometimes regulated with an off-axis regulator. That is, inkjet pen regulators are located at a site remote of the reciprocating carriage to which the pen is mounted.